Carbon fiber or C—C composites which are useful for instance in airplane braking systems are subject to oxidation and resultant weight loss (that is, loss of mass). Oxidative weight loss of such carbon composites is generally retarded by coating articles made of the carbon composites with an antioxidant coating.
U.S. patent application Ser. No. 09/518,013 (Golecki), filed 3 Mar. 2000, now U.S. Pat. No. 6,737,120 B1, relates to carbon fiber or C—C composites that are useful in a variety of applications. Golecki teaches methods of protecting such composites against oxidation by coating them with fluidized-glass type mixtures. The fluidized-glass mixtures are maintained as liquid precursors and are applied to components formed of carbon fiber or C—C composites. Once coated with the precursors, the coated C—C components are heat-treated or annealed for one or more cycles through a series of gradual heating and cooling steps. This creates glass coatings having thicknesses of about 1–10 mils. The thicknesses of the glass coatings may be varied by varying the composition of the fluidized glass precursor mixtures, the number of application cycles, and/or the annealing parameters.
The Golecki application teaches that the fluidized glass materials may comprise such materials as borate glasses (boron oxides), phosphate glasses (phosphorus oxides), silicate glasses (silicon oxides), and plumbate glasses (lead oxides). These glasses may include phosphates of manganese, nickel, vanadium, aluminum, and zinc, and/or alkaline and alkaline earth metals such as lithium, sodium, potassium, rubidium, magnesium, and calcium and their oxides, and elemental boron and/or boron compounds such as BN, B4C, B2O3, and H3BO3. By way of example, Golecki discloses a boron-containing liquid fluidized glass precursor mixture that includes 29 weight-% phosphoric acid, 2 weight-% manganese phosphate, 3 weight-% potassium hydroxide, 1 weight-% boron nitride, 10 weight-% boron, and 55 weight-% water.
The composites coated with boron-containing fluidized-glass type mixtures of Golecki—that is, coated articles comprising components made of carbon fibers or carbon-carbon composites annealed at very high temperatures, each component being covered by a glass coating of approximately 1–10 mil—are protected against uncatalyzed oxidation when the article is subjected to temperatures of 800° C. (1472° F.) or greater, for instance at 1600° F. (871° C.). However, these composites do not show good resistance to some types of catalytic oxidation. In particular, for instance, such boron-containing glass-coated composites have been found to be subject to significant oxidative weight loss after exposure to airport runway de-icer (potassium acetate).
U.S. patent application Ser. No. 09/507,414 (Walker and Booker), filed 18 Feb. 2000, now U.S. Pat. No. 6,455,159 B1, likewise relates to carbon-carbon composites and graphitic materials. The Walker and Booker application has as objectives the protection of carbon/carbon composites or graphites at elevated temperatures up to and exceeding 850° C. (1562° F.) and the reduction of catalytic oxidation at normal operating temperatures.
Walker and Booker achieve these objectives by employing a penetrant salt solution which contains ions formed from 10–80 wt % H2O, 20–70 wt % H3PO4, 0.1–25 wt % alkali metal mono-, di-, or tri-basic phosphate, and up to 2 wt % B2O3. Their penetrant salt solutions also include at least one of MnHPO4.1.6H2O, AlPO4, and Zn3(PO4)2, in weight-percentages up to 25 wt %, 30 wt %, and 10 wt %, respectively.
While the Golecki coatings and the Walker and Booker coatings do provide significant antioxidant protection for carbon composites, there remains a need for antioxidant coating systems that provide a high level of oxidation protection simultaneously in both high temperature and catalyzed oxidation conditions.